1. Technical Field
The present invention relates to a sheet transport device transporting a sheet from the upstream side to the downstream side in a transport direction, a recording apparatus including the sheet transport device, and a sheet transport method.
2. Related Art
An ink jet printer (hereinafter, “printer”) has been widely known as a recording apparatus recording on a sheet transported by a sheet transport device (hereinafter, briefly “transport device”). The transport device in the printer transports the sheet by a predetermined distance at an interval by controlling an amount of rotation of a transport roller based on the output result of a rotary encoder coaxially provided with the transport roller transporting the sheet in the sub scanning direction (transport direction).
The rotary encoder includes a disk shape encoder scale on which graduations are formed. The encoder scale rotates with the transport roller and obtains a phase of the transport roller corresponding to the graduations passing a sensor. Accordingly, if the center of rotation of the encoder scale is deviated from the center of the disk, causing eccentricity, an actual transport distance of the sheet varies periodically and continuously.
In a printer disclosed in JP-A-11-49399, the circumference of an encoder scale is divided into a plurality of sections, and a correction value (number of pulses) for each section is set in advance and stored. When the count of the encoder scale is a predetermined count in the corresponding section, a transport distance of a sheet is corrected by changing the amount of rotation of the transport roller, with the number of pulses corresponding to the correction value being supplied.
In the case of recent printers in which variations of sheet transport distance are reduced by improvement of assembling precision, an amount of deviation for one count of the encoder is very small. Accordingly, the count for one pulse that is the minimum unit of the correction amount becomes large and a highly accurate calculation may be needed. Since the count for the graduation of the encoder scale becomes large and the degree of precision is high, more storage areas may be required. Operation speed may be reduced because of the large count, and if the calculation is not performed precisely, correction may not be performed at a suitable time.